Differential Diagnosis
- Single most likely diagnosis
- Varicella-zoster virus: The patient's presentation of chest and upper back pain with an associated rash, particularly given the description and the patient's immunocompromised state due to methotrexate use, strongly suggests a diagnosis of herpes zoster (shingles). The rash typically follows a dermatomal distribution, which aligns with the description of left chest and upper back pain.
- Other Likely diagnoses
- Herpes simplex virus: Although less likely given the location and description of the pain and rash, herpes simplex virus could be considered, especially if the rash were more localized and not strictly dermatomal.
- Diabetic dermopathy: This condition is a possibility given the patient's history of type 2 diabetes, but it typically presents as multiple, small, brownish patches without the acute onset of pain.
- Drug side effect: Methotrexate can cause various skin reactions, but these are usually more generalized or differ significantly from the described dermatomal rash.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Staphylococcus aureus infection: While less likely given the description, a bacterial infection such as cellulitis or abscess could present with pain and rash. Missing this diagnosis could lead to severe consequences, including sepsis.
- Rare diagnoses
- Other rare viral infections or dermatological conditions could mimic the presentation but are less likely given the information provided. These might include other viral exanthems or autoimmune skin conditions exacerbated by the mixed connective tissue disorder or methotrexate therapy.